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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet/has below:

WeakSet.prototype.has() - JavaScript | MDN

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The has() method of WeakSet instances returns a boolean indicating whether an object exists in this WeakSet or not.

const weakset1 = new WeakSet();
const object1 = {};
const object2 = {};

weakset1.add(object1);

console.log(weakset1.has(object1));
// Expected output: true

console.log(weakset1.has(object2));
// Expected output: false

value

The value to test for presence in the WeakSet.

Returns true if an element with the specified value exists in the WeakSet object; otherwise false. Always returns false if value is not an object or a non-registered symbol.

const ws = new WeakSet();
const obj = {};
ws.add(window);

ws.has(window); // returns true
ws.has(obj); // returns false

// Storing a non-registered symbol
const sym = Symbol("foo");
ws.add(sym);
ws.add(Symbol.iterator);

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